Gestational diabetes is often the culmination of years of unrecognized and unmodified diabetes risk factors that lead to overt and occult clinical manifestations during pregnancy. Systematic reviews of older studies conclude that 35-60% women with gestational diabetes will develop type 2 diabetes at rates much greater than control groups who did not have glucose intolerance during pregnancy. The higher rates were in studies of particular ethnic groups in the U.S. Presently, in the literature, there are described new, more efficient methods of diabetes prevention in groups with a high risk of this disorder, which involve both, lifestyle modification and pharmacological therapies. Lifestyle intervention was found to reduce the incidence of type 2 diabetes by 58% and metformin by 31% as compared with placebo. Studies are needed for optimal postpartum and long-term health of women who have had GDM. Considerable recent evidence suggests that incretin-based therapies may be useful for the treatment of DM2 because continuous administration of glucagon-like peptide 1 (GLP-1) produces substantial improvements in glucose control and ß-cell function in subjects with type 2 diabetes. Inhibition of dipeptidyl peptidase-4 (DPP-4) increases the concentration of GLP-1 and may potentially delay disease progression in GDM considering the ß-cell function improvement in DM2 and ß-cell mass shown to increase in animal models. This study will examine if combination sitagliptin-plus metformin is more effective than metformin alone or placebo in improving metabolic parameters, specifically the impact on β-cell function, in at-risk women with a recent history of GDM.